Electronic timepieces are known which is equipped with a solar panel and a secondary battery and operates on electric energy that is generated by the solar panel and stored in the secondary battery. Among such electronic timepieces are ones which make a display for notifying a user of a state that the voltage of the secondary battery has become so low that the secondary battery needs to be charged.
Techniques relating to the present invention are disclosed in prior art references JP-A-224544 (corresponding to US 2008/0225647 A1) and JP-A-2008-224545 (corresponding to US 2008/0225648 A1). That is, they disclose techniques for displaying an amount of generated power in real time or displaying an accumulated electric energy generated or a duration of a timepiece operation in an electronic timepiece in which power is generated by self-winding and manual winding.
In secondary batteries, the battery voltage and the stored electric energy do not have a proportional relationship. At a stage that the stored electric energy is small, a reduction in battery capacity can be detected from a battery voltage because the battery voltage decreases at a relatively high rate as the secondary battery is used. However, at a stage that the battery capacity is in a medium range, it is difficult to estimate a battery capacity from a battery voltage because the battery voltage is kept almost constant.
The procedure that a reduction in battery capacity is detected from a battery voltage and a user is informed that the secondary battery needs to be charged is associated with the following problems. Since the battery capacity has already become so low that a transition to a sleep mode in which various functions of the timepiece are suspended may be made if a non-power-generation state continues for a certain period of time. If such an event occurs, it takes a long time to charge the secondary battery fully. That is, it is preferable that the user be informed a little earlier that the secondary battery needs to be charged, because this allows the user to cause, with a sufficient margin, a transition to an environment that enables power generation.
Measurement of generated power necessitates power consumption because, for example, it is necessary to cause a generated current to flow through a detection resistor. Since a generated current is not very large, it is difficult to use a detection resistor having so small a resistance that its power consumption is negligible. Therefore, even in the case where generated power is measured in such a manner that a user is notified with proper timing that the secondary battery needs to be charged, the energy consumption is large as long as the generated power is measured all the time.